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2/15/2011Harvard Bits2 US Telegraph “Network” in 1856

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2/15/2011Harvard Bits4 Size of switch grows as square of number of telephones Impractical to centralize switching as number of telephones grows

 Historically a telephone call is completed by setting switches so there is a continuous electric circuit from telephone to telephone  Pros: Dedicated line means uninterrupted service once circuit is completed 2/15/2011Harvard Bits5

 Number of calls limited by size and number of switches  Long telephone lines are “seized” by the call even if no one is talking, one party hangs up, etc.  Hard to utilize alternative routes if a switch along the principal path is overloaded 2/15/2011Harvard Bits6

 Alternative, used by the Internet  Break message into data packets of 1-2KB  Address the packets to their destination and serial number them so they can be reassembled at the other end  Let network figure out how to deliver them  Different packets of same message may take different routes 2/15/2011Harvard Bits7

 Extremely efficient use of network lines - whenever a link is not transporting a packet it is available for completely unrelated messages  Size of switch depends on actual data traffic, not on the number of simultaneous communications that might be happening 2/15/2011Harvard Bits8

 No performance guarantees possible, so hard to be sure real-time communications will work  Routing algorithms are not obvious, though if they can be made adaptive, the network could heal itself in case of localized catastrophes  Seems to require more intelligence at the edge of the network, while circuit switching requires intelligence in the core and can tolerate dumb devices at the edge 2/15/2011Harvard Bits9

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2/15/2011Harvard Bits11 Client Computers Web Server Server pop.fas.harvard.edu Server smtp.fas.harvard.edu downloadupload THE INTERNET

 Router in network core receives incoming packets and stores them in “buffer” (temporary storage)  Routes packets on outgoing links  May throw packets away if buffer is full 2/15/2011Harvard Bits12 Routing Table

 Routers are relatively dumb and rely on intelligence at the edge to compensate 2/15/2011Harvard Bits13 Packetize Add serial #s Add fingerprint Add destination address Insert into network BEST EFFORT Reassemble packets (Maybe) report missing packets (Maybe) report damaged packets Deliver to application Client application: , web browser, iTunes Server application

 IPv4: 32 bits written as 4 decimal numerals less than 256, e.g (UMich)  4 billion not enough  IPv6: 128 bits written as 8 blocks of 4 hex digits each, e.g. AF43:23BC:CAA1:0045:A5B2:90AC:FFEE:8080  At edge, translate URLs --> IP addresses, e.g. umich.edu -->  Authoritative sites for address translation = “Domain Name Server” (DNS)  In the network core, IP addresses are used to route packets using routing tables 2/15/2011Harvard Bits14

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 We treat IP addresses as Non-Personal Information  We reserve the right to share Non-Personal Information with affiliates and other third parties. 2/15/2011Harvard Bits16

 ICANN = Internet Corporation for Assigned Names and Numbers  A US nonprofit … but it’s a long story. 2/15/2011Harvard Bits17

 Routers do not know what the bits in the packets represent  Do not know if they are , streaming video, html web pages  Do not know if they are encrypted or unencrypted  You can invent your own new service adhering to IP standards  Gain Internet’s best-effort service  and possibility of undelivered packets 2/15/2011Harvard Bits18

 Packet size (1.5 KB max) a compromise  Small enough that they can be “handled” quickly and with relatively low odds of being damaged  Large enough that packaging does not outweigh the contents or “payload” 2/15/2011Harvard Bits19

 Smallish packets also make better use of the network since later packets can leave before earlier packets arrive 2/15/2011Harvard Bits

 Smallish packets also make better use of the network since later packets can leave before earlier packets arrive 2/15/2011Harvard Bits

 Smallish packets also make better use of the network since later packets can leave before earlier packets arrive 2/15/2011Harvard Bits

 Smallish packets also make better use of the network since later packets can leave before earlier packets arrive 2/15/2011Harvard Bits

 Smallish packets also make better use of the network since later packets can leave before earlier packets arrive 2/15/2011Harvard Bits

 Smallish packets also make better use of the network since later packets can leave before earlier packets arrive 2/15/2011Harvard Bits

 Smallish packets also make better use of the network since later packets can leave before earlier packets arrive 2/15/2011Harvard Bits

 Smallish packets also make better use of the network since later packets can leave before earlier packets arrive 2/15/2011Harvard Bits

 Smallish packets also make better use of the network since later packets can leave before earlier packets arrive 2/15/2011Harvard Bits

 Smallish packets also make better use of the network since later packets can leave before earlier packets arrive 2/15/2011Harvard Bits

 Smallish packets also make better use of the network since later packets can leave before earlier packets arrive 2/15/2011Harvard Bits

 Store and Forward delays would add up if entire message had to be buffered at every router 2/15/2011Harvard Bits

 Store and Forward delays would add up if entire message had to be buffered at every router 2/15/2011Harvard Bits

 Store and Forward delays would add up if entire message had to be buffered at every router 2/15/2011Harvard Bits

 Store and Forward delays would add up if entire message had to be buffered at every router 2/15/2011Harvard Bits

 Store and Forward delays would add up if entire message had to be buffered at every router 2/15/2011Harvard Bits

 Store and Forward delays would add up if entire message had to be buffered at every router 2/15/2011Harvard Bits

 Creates logical connection between two machines on the edge of the network  Connected machines seem to have a circuit connecting them even though they do not tie up the network  Provide reliable, perfect transport of messages, even though IP may drop packets  Regulates the rate at which packets are inserted into the network 2/15/2011Harvard Bits37

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2/15/2011Harvard Bits “3-Way Handshaking”

2/15/2011Harvard Bits “3-Way Handshaking”

2/15/2011Harvard Bits “3-Way Handshaking”

2/15/2011Harvard Bits “3-Way Handshaking”

2/15/2011Harvard Bits “3-Way Handshaking”

2/15/2011Harvard Bits “3-Way Handshaking”

2/15/2011Harvard Bits * “3-Way Handshaking”

2/15/2011Harvard Bits * “3-Way Handshaking”

2/15/2011Harvard Bits * “3-Way Handshaking”

2/15/2011Harvard Bits * “3-Way Handshaking”

2/15/2011Harvard Bits * “3-Way Handshaking”

2/15/2011Harvard Bits * “3-Way Handshaking”

2/15/2011Harvard Bits * “3-Way Handshaking”

2/15/2011Harvard Bits * “3-Way Handshaking”

2/15/2011Harvard Bits * “3-Way Handshaking”

2/15/2011Harvard Bits * “3-Way Handshaking”

2/15/2011Harvard Bits * “3-Way Handshaking”

2/15/2011Harvard Bits * “3-Way Handshaking”

2/15/2011Harvard Bits “Virtual Circuit” now established between two hosts though the routers in between are not aware of it and the same path need not be followed by all packets

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2/15/2011Harvard Bits65 ACK

2/15/2011Harvard Bits66 ACK

2/15/2011Harvard Bits67 ACK ACK2

2/15/2011Harvard Bits68 ACK ACK2

2/15/2011Harvard Bits69 ACK ACK2 2

2/15/2011Harvard Bits ACK2

2/15/2011Harvard Bits ACK2

2/15/2011Harvard Bits ACK2

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2/15/2011Harvard Bits TIMEOUT

 Used for real-time applications (e.g. streaming audio and video) where timing is essential but perfect delivery is not 2/15/2011Harvard Bits84

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 Both TCP (guaranteed delivery) and UDP (fast delivery, no guarantees) use the lower-level Internet Protocol in the “link layer”  But TCP and UDP know nothing about links, routing, etc. All that knowledge is embedded in IP 2/15/2011Harvard Bits94

2/15/2011Harvard Bits95 Higher Level Protocols Lower Level Protocols